1. Field of the Invention
The present invention relates to electric wires or cables.
2. Description of the Prior Art
Electric wires or cables having dielectric layers of polyolefins such as polyethylene, cross-linked polyethylene and the like are in wide use. Such polymers have in recent years been improved so that they are able to withstand high applied voltages. One important problem at present with such electric wires and cables is that they undergo a gradual degradation with time during under continuous applied voltages, resulting in a reduction in their capability to withstand applied voltages. Since the service life of electric wires or cables per se is as long as thirty years, the initial thickness of dielectric layers therefor must be increased over initial requirements with present designs so as to compensate for the above reduction in their ability to withstand applied voltages with the passage of time. On the other hand, recent urban development have required increased power transmission capacity for electric wires or cables while keeping volume as small as possible. This has led to the difficult problem, with electric wires or cables having a dielectric layer essentially consisting of a polyolefin, of increasing the transmission voltage without increasing the thickness of the dielectric layer.
Various approaches to overcome the above problem have been considered, including preventing the occurrence of the above described degradation, whereby the decrease in capability to withstand applied voltages during a thirty year service life need not be taken into account, and the requirements of urban areas regarding high transmission capability and low volume can be met.